The present invention relates generally to power conversion systems and deals more specifically with a boost-buck rectifier bridge circuit topology with diode decoupled boost stage.
Conventional boost-buck rectifier circuit topologies such as illustrated in FIG. 1 are generally well known and provide limited power to the load due to the current handling limitations of the switching device employed and as shown in dash line box 12, 14 and the switching device's antiparallel diode as shown in the dash line box 16, 18, respectively. The switching device 12, 14 is typically a high power switching transistor or power MOSFET. An Insulated Gate Bipolar Transistor (IGBT) offers a number of advantages over conventional switching devices and can be packaged with an integral antiparallel diode to form a single semiconductor module. The IGBT further is desirable rather than a power MOSFET in high voltage, hard switching applications because an IGBT has switching characteristics similar to a power MOSFET while maintaining its superior conduction ability. IGBT advantages over power MOSFET's and other power switching devices include lower conduction losses and smaller die area for the same output power. The smaller die area in IGBT's results in lower input capacitance, higher switching speeds and lower cost. Consequently it would be desirable to use IGBT's in boost-buck rectifier bridge circuit topologies if the current carrying limitations of the switching devices in the boost stage could be overcome.
In both the buck and boost modes, the main output load current must flow through the diodes 16, 18 during alternate half cycles of the generator voltage. Because the diodes are an integral part of the switching devices 16, 18, respectively, the diodes exhibit a high conduction loss due to the large forward voltage drop developed across the diode. Accordingly, the output power capable of being produced by the conventional boost-buck rectifier bridge circuit topology is severely limited by the thermal capability of the switching device antiparallel diode. Additional parallel diodes could in theory be used, however the additional diodes would have to be closely matched for proper current sharing. The use of matched diodes is not desirable and generally not feasible due to increased cost, increased component count, and complexity. Furthermore, in the event of a diode failure, the replacement of the failed diode must be with a matched diode. From a practical standpoint, both the failed diode and the remaining diode of the matched pair would have to be replaced with a known matched pair to ensure proper current sharing.
The above referenced and other power limitations associated with conventional boost-buck rectifier bridge circuit topologies due to the current handling limitations of the switching device and antiparallel diode is solved with the boost-buck rectifier bridge circuit topology of the present invention by decoupling the boost stage and the IGBT antiparallel diodes from the main current loop so that the load current does not flow through the antiparallel diodes.
A further advantage of the present invention is that the voltage generator can provide substantially more generator current than is possible in a conventional boost-buck rectifier bridge circuit topology so that the internal inductance of the voltage generator windings is sufficient to limit the generator current to a safe amplitude without an additional separate inductor which is required with conventional boost-buck rectifier bridge circuit designs.
A yet further advantage of the present invention is the provision of a high power boost-buck rectifier bridge circuit topology that has a lower component count than conventional boost-buck rectifier bridge circuit topologies.